Henry Kuttner & C L Moore's "Android" (as by C H Liddell) (novelette, paranoia, free): Androids are quietly replacing humans!
The idea is not new - a conspiracy of things non-human to quietly impersonate humans, sometimes killing the originals they're replacing; see, e.g., John Campbell's "The Brain Stealers of Mars" (read online), or Philip K Dick's "The Father-Thing" (read online). Stylistically, there is some similarity with Kuttner's own "Don't Look Now" (download) about a man's paranoia that Martian imposters among humanity are driving our destiny.
We also see here a favorite theme of Kuttner: children think in ways that are fundamentally different from adults.
Story summary.James Philips Bradley has incontrovertible evidence that his boss, Arthur Court, is an android rather than human! In fact, his work has convinced him that a sizable number of people he sees around are actually androids. And he thinks they're working towards eventually replacing humanity, though their numbers may not be that big today.
This is the story of his quest (to save humanity). And its strange climax.
Fact sheet.First published: F&SF, June 1951,
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Related: Stories of Henry Kuttner, C L Moore (as by C H Liddell); fiction from 1950s.